Classical endoscopes such as can be used, for example, for minimally invasive surgery, guide the image by means of rod lenses from the intracorporeal objective to the extracorporeal eyepiece. As a result of the rod lenses, the system is rigid and limited in optical quality. Modern video endoscopes use a camera chip in the endoscope tip. Such an endoscope is disclosed in U.S. Pat. No. 7,365,768 B1. This has a rigidly disposed lens in front of the camera chip. An adjustment of the focal length of the lens is not possible.
DE 196 18 355 C2 discloses a linear drive which can be integrated in endoscopes for adjusting the focal width of a lens system. For this purpose a permanent magnet is moved as an armature inside a stator coil. However, as a result of the large mass of the permanent magnet, the linear drive is sluggish. The relationship between the coil current and the armature position is not single-valued and necessitates an additional displacement sensor with positional regulation.
DE 37 17 872 C2 discloses a drive having an armature and a stator for a lens system in video cameras. The armature consists of two iron sleeves which are interconnected by a support for receiving the lens system. The stator has two coils and a single annular permanent magnet for generating the magnetic fields required for the movement between the coils. The complex structure of the drive can be readily implemented in video cameras having lens diameters in the centimeter range but is not scalable to the size required for endoscope applications in the millimeter range.
DE 103 23 629 A1 discloses a moving field linear motor which includes at least three stator coils. A phase-shifted current supply to the coils produces a moving field which effects a displacement of the armature with axial permanent magnets. An expensive control circuit is required to produce the moving field.
Known from DE 10 2008 038 926 A1 is a linear drive including two axially polarized permanent magnets in the armature. The armature is deflected by the current supply to the stator coils in the axial direction. In addition, the stable positions of the armature are achieved by the pole piece mounted in the stator so that a continuous displacement of the armature in a cladding tube is rendered possible. A disadvantage here is the dependence of the stroke and the adjusting forces on the soft-magnetic stator pole pieces, with the result that a high precision is required in the manufacture and assembly of these parts.